The invention relates to a method for determining the spacing of two reference points which are movable relative to each other between a lower spacing limit and an upper spacing limit. The invention relates especially to a method for determining the spacing of a vehicle body of a motor vehicle relative to a fixed reference point utilizing a sensor having a measuring range subdivided into several sequential measurement value intervals. The same signal interval is assigned to each measurement value interval. The method is based on the following steps, namely: positioning the sensor in a defined position to the two reference points in such a manner that all possible spacings of the reference points are assigned precisely to the measurement values of one measurement value interval; the sensor indicates a lower limit signal in the lower spacing limit and indicates an upper limit signal in the upper spacing limit; the sensor indicates a signal between the lower limit signal and the upper limit signal when the spacing of the reference points lies between the lower and upper spacing limits; and, a conclusion is clearly drawn as to the spacing of the reference points from the signal indicated by the sensor.
Modern motor vehicles often have a level control system with air springs at least on the rear axle. With the aid of this system, the level of the vehicle body is adjustable relative to a fixed reference point. Such level control systems afford especially the advantage that the elevation of the vehicle body relative to a fixed reference point can be precisely adjusted independently of the loading condition of the motor vehicle. Furthermore, by charging the air springs with pressurized air, it is possible to lift the vehicle body, for example, in order to impart a greater ground clearance to the vehicle on uneven roadways. Furthermore, the vehicle body of the motor vehicle can be lowered by releasing air from the air springs, for example, in order to facilitate a loading of the vehicle. The vehicle body is therefore movable between a maximum elevation wherein the vehicle body assumes the greatest spacing to a fixed reference point and a minimum elevation wherein the vehicle body assumes the lowest spacing to a fixed reference point.
The level control system requires elevation sensors for controlling the elevation of the vehicle body relative to a fixed reference point. With the aid of the elevation sensors, the elevation of the vehicle body can be determined relative to a fixed reference point. Rotational angle sensors are conventionally used as elevation sensors and known per se. The rotational angular range of the rotational angle sensors is subdivided into sequential rotational angle intervals. Each rotational angle interval is assigned the same signal interval in a characteristic field (such rotational angle sensors are available in the marketplace in the form of vehicle level sensors, for example, as are available from Hella KG of Germany). Each of the rotational angle sensors is built into the vehicle in such a manner that all possible elevations of the vehicle body relative to a fixed reference point are as precisely assigned to the rotational angles of a rotational angle interval and so that the rotational angle sensor moves exclusively between a lower rotational angle and an upper rotational angle. In this case, the signal, which is indicated by the rotational angle sensor, moves exclusively between a lower limit signal and an upper limit signal of a characteristic field. With corresponding programming of the control unit, it is therefore possible to clearly draw a conclusion as to the elevation of the vehicle body relative to a fixed reference point from the signal which a rotational angle sensor indicates.
With the above-mentioned arrangement of a rotational angle sensor, the elevation of the vehicle body of a motor vehicle can be measured simply and precisely relative to a fixed reference point. However, it can happen that the sensor is built in imprecisely or component tolerances add unfavorably so that the sensor assumes rotational angles from two rotational angle intervals during a runthrough of the vehicle body through the entire elevation range (this is characterized in the following as a characteristic field overshoot). A correct assignment of the signal, which is indicated by the sensor, to a vehicle elevation is then no longer easily possible and can lead to a defective control as will be explained hereinafter with reference to the drawings.
It is an object of the invention to provide a method with the aid of which the spacing of two reference points, which are movable relative to each other between a lower spacing limit and an upper spacing limit can still be determined correctly by means of a sensor as described above when the location of the built-in sensor deviates from a pregiven built-in location therefor. The above spacing is especially the spacing of a vehicle body of a motor vehicle relative to a fixed reference point.
The method for determining the distance of two reference points, which are movable relative to each other between a lower distance limit and an upper distance limit, with a sensor having a measuring range subdivided into several successive measurement value intervals, the sensor outputting a signal with each of the measurement value intervals having the same signal interval assigned thereto, the method being based on the following: the sensor is positioned in a defined position to both of the reference points in such a manner that all possible distances of the reference points correspond to the measurement values of one of the measurement value intervals; the sensor indicates a lower limit signal at the lower distance limit and an upper limit signal at the upper distance limit; the sensor indicates a signal between the lower and upper limit signals when the distance of the reference points lies between the lower and upper distance limits; and, a conclusion is drawn from the signal indicated by the sensor as to the distance of the reference points; the method including:
(a) the following steps for calibrating the sensor: mounting the sensor as close as possible to the defined position relative to the two reference points; and, within at least one distance between the reference points, determining the difference of a signal, which a sensor would indicate if disposed at the defined position, and a signal (S) indicated by the sensor and determining an offset (O) from the difference; and,
(b) the following additional steps for determining the distance of the two reference points: adding the offset (O) to the signal (S) indicated by the sensor to form a summation signal (SS=S+O); assigning the distance of the reference points to the summation signal (SS), which distance would be assigned to the same equally large signal, which a sensor would indicate if disposed at the defined position, when the summation signal (SS) lies between the lower limit signal Smin and the upper limit signal Smax; determining the distance of the reference points from the summation signal (SS) as follows when the summation signal (SS) lies outside of the signal interval from the lower limit signal Smin to the upper limit signal Smax: when the summation signal (SS) lies above the upper limit signal Smax, assigning a distance of the reference points to the summation signal (SS) which lies in the vicinity of or corresponds to the lower distance limit (hmin); and, when the summation signal (SS) lies below the lower limit signal (Smin), assigning a distance of the reference points to the summation signal (SS) which lies in the vicinity of or corresponds to the upper distance limit (hmax).
As to a spacing, which lies in the vicinity of the lower spacing limit, it is understood to mean a spacing to the lower spacing limit which can lie between zero and ⅕ of the distance between the spacing limits. Correspondingly, a spacing which lies in the vicinity of the upper spacing limit is understood to be a spacing to the upper spacing limit which can lie between 0 and ⅕ of the distance between the spacing limits (for example, the distance between the spacing limits amounts to 100 mm; a spacing then lies in the vicinity of the lower spacing limit when it is between 0 and 20 mm away from the lower spacing limit).
The advantages achieved with the invention are especially seen in that the elevation of a vehicle body of a motor vehicle can even be determined with the aid of the above-mentioned sensors when one of the sensors is imprecisely built in because a characteristic field overwrite is detected and therefore an erroneous interpretation of the sensor signal is avoided. In this way, a position correction of a sensor is not necessary even when the sensor is not at the exact position. Rather, a fictive position correction is undertaken by considering the offset which is added to the signal indicated by the sensor. A further advantage of the invention is seen in that the fictive position correction of a sensor, which is imprecisely built in, is possible via the determination of a single offset. This offset can, for example, be already determined without great complexity and therefore cost effectively, for example, during the manufacture of the motor vehicle with a calibration of each sensor. A further advantage is seen in that the method of the invention applies to sensors which are available as cost effective standard components. In this way, a level control system with which the method of the invention is carried out is not more expensive relative to conventional level control systems equipped with such sensors.
According to another feature of the invention, wherein the signal, which is indicated by the sensor, is compared to an ideal signal which the sensor would provide if located in the defined position, the spacing or distance of the two reference points lies in a range wherein the characteristic line, which is generated by the sensor, runs substantially linearly. The advantage of this embodiment is seen in that the offset can be very easily and precisely determined.